It has been recognized that during the aqueous suspension polymerization of unsaturated monomers unwanted aqueous phase polymerization can occur resulting in polymer fouling of the polymerization reactors. Besides deposition of polymer on the internal surfaces of the reactors, the development of turbid or cloudy aqueous phase solutions from aqueous polymer formation can render the aqueous phase unsuitable for reuse or recycle.
Aqueous phase polymerization is especially troublesome when the suspension polymerization is conducted under an inert atmosphere, i.e., little or no oxygen is allowed in the reaction environment. The magnitude of aqueous phase polymerization will usually vary depending upon the solubility of the monomer or organic phase in the aqueous phase. Even monomers considered to be relatively water insoluble, e.g., styrene, have some finite water solubility, albeit small, which can lead to aqueous phase polymerization. During the polymerization the aqueous phase solubilized monomer encounters free radicals present in the aqueous phase resulting in aqueous phase polymer formation; this can be a significant problem in both suspension and emulsion polymerization processes.
A variety of aqueous phase inhibitors have been employed to address the problem of unwanted aqueous phase polymerization, but use of these inhibitors has not been entirely successful due to deficiencies associated with each system. For example, aqueous phase inhibitors that have some monomer phase solubility or partition into the monomer phase during the polymerization can retard the desired monomer polymerization rate resulting in inferior polymer properties. Other types of aqueous phase inhibitors require the presence of oxygen to be fully active as inhibitors and, since oxygen is generally removed to allow the desired polymerization to proceed smoothly, these inhibitors have found limited use. Some inhibitors, in order to be functional in the aqueous phase, require adjustment of the acidity or alkalinity in order to solubilize the inhibitor into the aqueous phase; the pH conditions necessary for solubilizing these inhibitors are not always compatible with other requirements of the desired monomer polymerization.
Other approaches to solving the polymer fouling problem during suspension polymerizations include treating the internal reactor surfaces to render the surfaces less susceptible to becoming fouled by polymer buildup. For example, U.S. Pat. No. 4,438,242 discloses a process for treating the internal steel surfaces of polymerization reactors with sulfur trioxide to retard polymer fouling during suspension polymerizations.
The present invention seeks to overcome the deficiencies of prior methods used to reduce aqueous phase polymerization during suspension polymerization by providing a process that does not detrimentally affect the desired monomer polymerization rate, does not require the presence of oxygen and does not require the special pretreatment of reactor vessels.